There are multiple versions of WorldCat. I STRONGLY suggest using FirstSearch.
General Tips:
I also very strongly suggest using the Advanced Search tab to utilize all WorldCat's excellent features.
Many of the features in the advanced search are self-explanatory, such as the ability to keyword search, author search, title search, subject search.
1. Use the Audience Filter under "subtype limits"
If you are getting history books aimed at school students, limit to non Juvenile.
OR if you want to use children's books as Primary Sources, limit to Juvenile books and limit publication date to the period of interest (Ex: "Civil war" as a subject search, limited to 1865-1890, limited to Juvenile will brings you childrens books about the Civil war written in the late 19th century)
2. Use the Any Content box to limit to fiction or nonfiction materials
Are you getting too many terrible fiction books about the Civil War? Limit to non fiction.
OR
Do you want to use novels and fictional works as primary sources? Limit to fiction and limit to a time period of interest. Use keywords and/or subject searches to find materials. Many of the results are either scanned online in HathiTrust or googlebook, or can be ILL'd.
3. Limit to format
In some cases, you may want to limit your search to a specific format. Most often, this is useful trying to find Microfilm reprints of archival collections. The manuscript option can also be used to find archival materials based on your topic
4. Limit to languages
You can use the language box to limit to specific languages. NOTE this is not all the options.
You can also also use the "language phrase" option in the search boxes for languages not listed in the standard list. For example, a search for Klingon* in the search box as a language phrase brings up books written in Klingon.